1. Field of the Invention
The invention relates to an energy monitoring device, and particularly for monitoring the energy of an extreme ultraviolet radiation source emitting around 11-15 nm.
2. Discussion of the Related Art
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems operating around 248 nm, as well as the following generation of ArF-excimer laser systems operating around 193 nm. Molecular fluorine (F2) lasers operating around 157 nm are being developed for use in Vacuum UV (VUV) lithographic processing systems. Extreme UV (EUV) or soft x-ray radiation sources for EUV lithography emitting 11 nm-15 nm photon beams are currently also being developed.
EUV radiation sources have an advantageous output emission beam including 11-15 nm wavelength photons having photon energies in a range around 90 eV. This short wavelength is advantageous for photolithography applications because the critical dimension (CD), which represents the smallest resolvable feature size producible using photolithography, is proportional to the wavelength. This permits smaller and faster microprocessors and larger capacity DRAMs in a smaller package.
A promising technique for producing EUV lithography beams uses a pair of plasma pinch electrodes for driving a preionized azimuthally symmetrical plasma shell to collapse to a central axis. A power supply circuit supplies a high energy, short duration pulse to the electrodes, wherein several kilovolts and up to at a hundred kA or more are applied over a pulse duration of less than a microsecond. A Z-pinch electrode arrangement generates a current through the plasma shell in an axial direction producing an azimuthal magnetic field that provides the radial force on the charged particles of the plasma responsible for the rapid collapse.
The excimer and molecular fluorine lithography lasers, mentioned above, emit laser beams using a gas discharge for creating a population inversion to a metastable state in the laser active gas, and a resonator for facilitating stimulated emission. It is not yet completely clear what radiative mechanism is responsible for the axial, high energy photon emission in plasma pinch EUV sources. The collapsing shell of charged particles of the plasma have a high kinetic energy due to their velocities in the radial direction. The rapid collapse of the shell results in collisions between all portions of the incoming shell at the central axis with radially opposed portions of the incoming shell.
The high kinetic energies of the particles are abruptly transformed into a hot, dense plasma which emits x-rays. A high recombination rate concentrated in the azimuthal direction due to the plasma being particularly optically dense in the azimuthal direction has been proposed (see, Malcolm McGeoch, Radio Frequency Preionized Xenon Z-Pinch Source for Extreme Ultraviolet Lithography, Applied Optics, Vol. 37, No. 9 (Mar. 20, 1998), which is hereby incorporated by reference), and population inversion resulting in spontaneous emission and predominantly azimuthal stimulated emission, and bremsstrahlung resulting from the rapid radially deceleration of the charged particles of the collapsing plasma, are other mechanisms of high energy photon emission.
In the past, i.e., prior to the investigations leading up to the present application, very little was known about the behavior of EUV-photodetectors under long-term 157 nm laser pulse radiation exposure. In addition, until now, no reliable energy monitor for detecting the pulse energy of an extreme ultraviolet beam having a wavelength around 11-15 nm has been available. For detecting 193 nm and 248 nm excimer laser radiation, UV-photodetectors such as the International Radiation Detectors, Inc. (IRD) UVG 100 (Si photodiode with oxinitride cover) or the Hamamatsu S 5226 or S1226 have been used typically as energy monitor detectors. However, these detectors may strongly degrade under exposure to EUV photon radiation.
It is therefore desired to have a reliable photodetector for monitoring pulse energies and/or another parameter of an extreme ultraviolet radiation source.